Cascade amplifier using transformer coupling between thermionicvacuum tubes



Jan. 8, 1929. r 1,697,923

C. A. L. LOWRY ET AL CASCADE AMPLIFIER USING TRANSFORMER COUPLING BETWEEN THERMIONIC VACUUM TUBES Filed Nov. 50, 1927 INVENTEIRE oancoc uxgw Patented, Jan. 8, 1929.

UNITED STATES 1,697,923- PATENT OFFICE.

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TARIO, CAN ADA.

CASCADE AMPLIFIER USING TRANSFORMER COUPLING BETWNEN THERKICNIC- VACUUM TUBES.

Application filed November 80, 1987. Serial No. 238,796.

When constructing cascade amplifiers it is customary to introduce means for reventing the development of uncontro led feed-backs which wouldresult in poor general operation. Toward this end most mod-- ern amplifiers of two or more stages are enclosed in metal housings which often take the form of a c'atacomb, Le. a box divided into compartments, into each of which is fitted the 0011 or in some cases the whole stage including the tube and associated tuned circuit.

This practice is quite common particularly in the case of superheterodyne receivers wherein the intermediate wave amplifier is enclosed wholl or in part within a shielding catacomb. guch amplifiers are generally operative on a fixed frequency and are tuned, as to each stage, once and for all, at the factory. It is very important. that these stages be accurately tuned, and that the amount of feed back with its resultant regenerative efiect toward greater amplification be under complete control so as to be neither so great as to cause unstable operation or so little as to result in poor efi iciencyr To construct an amplifier that will havejust,enough interstage coupling and at the same time have accurately tuned stages so as toproduce a relatively ideal result, is a simple laboratory operation but becomes extremely difiicult of duplication on a factory production basis.

For example, when winding 8. large number of inductance coils for operation in, say, a four stage type of amplifier tuned for operation at a fixed frequency -of, say, 192.5 kilocycles, there are many things that must be watched.

1. The coil formers must all be of exactly the same size.

2. The coils must all have exactly the same number of turns.

3. Care must be taken when placing the coils in the catacombs to have them accurateform gauge and the insulation. on the wire, or between turns and layers must be of ab-.

If all these conditions could be met in production it would'not be necessary to resort to any dodges to efiect the development of satlsfactor amplifiers on a commercial basis. The rst three of these conditions can be met satisfactorily but the fourth condition cannot. It is not ossible at this time toymake inductance coi s anywhere nearly alike, due principally to variations in gauge of the wire and in thickness of insulation. Nor can coils be matched to a suflicient degree of accuracy by any method now known so that when placed in service they will function without at least some adjustment. The primary purpose of. this invent1on is to provide cheaply a means for uickly making the necessary final slight a justment to regulate feed back and to effect pertect tuning after the amplifying device has been actually placed in operation.

We attain our object by providing a cellular metal shield with a movable metal cover constructed and arranged so that the air aps between the cover and the division wall s of the cells may be varied to vary the feed back from coil to coil, and so that the tunmg of lndividual amplifiers may be adjusted by varying the distance of the cover from the inductance of the amplifier.

The invention is illustrated in the accompanying drawings in which Flg. 1 is a lon itudinal section of a metal -shield formed with five cells; and

Fig. 2 a cross section of the same.

. In the drawings like numerals of reference indicate corresponding parts in the different figures. V

In the embodiment of the invention illusand of an arched form. This cover when set in place and supported by its edges allows an air-gap to exist between adjoining cellsof the shield and so permits considerable fe'e'djbackfbetween the coils. Ordinarily the coverwill rest on the partitions as shown.

A means is then rovided for varying these air-gaps. A heavy ar 4 extending lengthwise along the centre of the cover is held at its ends by threaded legs extending down through the cover and through the bottom of the shield and movable from points outside the latter. Usuallythe ends of the legs will be threaded and nuts 5 screwed thereon. By turning the nuts on these legs, thus moving them up or down, the cover can be more or less flexed one way or the other so as to open or close the air-gaps.

In addition to this a further effect is produced. When an inductance is enclosed within a metal casing its electrical characteristics at high frequency are somewhat affected b the distances of the metal walls from the s des and ends of the coil. In other words,

' tion of tuning adjustments within narrow limits. Thus by jugglin the two adjusting nuts on the bottom of t e shield an experienced operator can obtain perfect cascading of an intermediate amplifier.

For the passage of connecting wires the sides of the cover are provided with notches 6.

The original area of the air-gap will vary in different designs as it depends upon coil characteristics and will be larger in large shields enclosing small coils than in small shields enclosing large coils. It will be smaller when coils operate at high frequeneies, and will be larger when used with coils operating at comparatively. low frequencies.

It must be clearly understood that the constructions described will usually be of value only, when the coils are matched within narrow limits, as the adjustment effected by varying the position of the cover is ver fine an itself operates over very narrow imits in ordinary practice.

With mechanical modifications this arrangement could also be applied to cascade amplifiers of the tuned radio frequency t tit at we claim as our invention is: 1. A metal shield for cascade amplifiers using transformer coupling comprising a plurality of open topped cells arranged side gap between the cover and the by side and separated by a partition wall or walls; a cover for the cells held in spaced relationship to the partition; and means for varying the position of the cover relative to the partition to vary the air-gap between the cover and the partition.

2. A metal shield for cascade amplifiers using transformer coupling comprising a plurality of open topped cells arranged side by side and separated by a partition wall or walls; a flexible cover for the cells held in spaced relationship to the partition; and means for flexing the cover to vary the airartition.

3. A metal shield for cascade amplifiers usin transformer coupling comprising a box divi ed into a plurality of cells by a plurality of partitions; a transversely arched flexible metal plate supported by its edges over the partitions so as to leave air gaps between its under surface and the partitions; and means for flexing the cover to vary the air-gaps between the cover and the partitions.

4. A metal shield for cascade amplifiers usin transformer coupling comprising a box divi ed into a plurality of cells by a plurality of partitions; a transversely arched flexible metal plate supported by its edges over the partitions so as to leave air gaps between its under surface and the partitions; and means for flexing the cover to vary the air-gaps between the cover and the partitions comprising a bar engaging the back of the flexible cover, and means engaging the bar adjacent opposite ends to move it to flex the cover.

5. A metal shield for cascade amplifiers using transformer coupling comprising a box divided into a plurality of cells by a plurality of partitions; a transversely arched flexible metal plate supported by its edges over.the partitions so as to leave air-gaps between its under surface and the partitions; and means for flexing the cover to vary the air-gaps between the cover and the partitions comprising a bar engaging the back of the flexible cover, legs connected with the bar and extending down through the bottom of the box, and iioeans for adjusting the legs relative to the 6. A metal shield for cascade amplifiers using transformer coupling comprising a pinrality of open topped cells arranged side by side and separated by a partition wall or walls; a cover for the cells held in spaced relationship to the partition, the edges of the cover being notched to provide openings for the passage of connecting wires; and means for varying the position of the cover relative to the partition to vary the air-gap between the cover and the partition. I

Signed at Toronto, Can. this 15th day of November 1927.

' CHARLES A. L. LOWVRY.

WILLIAM J. WATLING. 

